Case-control studies: a sequential approach.

A sequential approach to the design of a matched pair case-control study is proposed as an alternative to fixed sample plans when information on case-control pairs is acquired sequentially in time. The method described is that of Wald using the Sequential Probability Ratio Test (SPRT) for comparing two binomial populations. The test is an open, non-truncated procudure. Several tables are presented for the average sample numbers needed under the sequential plan and fixed sample plan for selected Type 1 and 2 errors, alternative relative risks of interest and exposure probabilities in case and controls. A hypothetical example is presented to illustrate the use of the method and discussion is given as to the possible advantages and disadvantages of the sequential approach to case-control studies.